1. Field of the Invention
The invention relates to a device for converting a rotational movement into a reciprocating movement, in particular cam control, valve timing gear for internal combustion engines of motor vehicles or the like. The device has at least one cam element which is disposed on a driven support shaft and has an eccentric control surface and having a cam follower element, in particular a valve tappet or the like, which can be displaced or pivoted by the cam element. The cam element is rotatably disposed in a flexible enclosing element which is connected to one end of the cam follower element in a manner enabling it to move in a plane which is perpendicular with respect to the axis of rotation of the cam element.
Since customary valves of internal combustion engines require, for them to be closed, restoring springs which have to apply considerable forces, constrained guides have also already been proposed, these requiring weaker restoring springs or rendering them superfluous. One particular embodiment of a constrained guide of this type can be found, for example, in German published patent application DE 37 00 715 A1. There, the cam element is surrounded in a loosely adjacent manner by a flexible enclosing element that is connected to the valve actuating element. The cam element therefore revolves in the enclosing element.
I have previously described various developments of the foregoing type of constrained guide. See, for example, my international publications WO 01/12958 A (US 2002/0073947 A1) and WO 01/12959 A (US 2002/0185092 A1). When these enclosing elements are used, friction occurs between the circumferential surface of the cam element and the inner surface of the enclosing element, and it has therefore also been proposed to insert a friction-reducing medium between the circumferential surface of the cam element and the enclosing element via radial ducts in the cam element.
Since the enclosing element is subjected to relatively high tensile forces by the reciprocating cam follower element particularly when the push-off acceleration is braked, that part of the enclosing element which lies opposite the connecting region is pressed fixedly against the circumference of the cam element. Conversely, that part of the enclosing element which encloses the connecting region is exposed, shortly before it returns into the starting position, to correspondingly high compressive forces, since the restoring acceleration is braked, and is pressed onto the circumference of the cam element. In both cases, outlet openings situated in these regions are tightly closed by the enclosing element, and a very high pressure would be required to feed in the lubricating medium. For example, there is a pressure of 2 to 5 bar in conventional cylinder heads, and at least 10 times the pressure would have to be able to be applied in order to push the enclosing element away from the circumference and to allow the medium to emerge. (The values of this example relate to lubrications using oil). Only partial lubricant films are produced, and a mixed friction occurs, the coefficient of friction of which is not smaller than 0.1.